This application claims the priority of German patent document 103 61 286.6, filed Dec. 24, 2003 (PCT International Application No. PCT/EP2004/013604, filed Dec. 1, 2004), the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for regenerating a nitrogen oxide storage catalytic converter arranged in an exhaust pipe of an internal combustion engine.
German patent document DE 101 13 947 A1 discloses a method for regenerating a nitrogen oxide storage catalytic converter of the generic type. Nitrogen oxide storage catalytic converters are used in particular in motor vehicles which have an internal combustion engine which can be operated with an air/fuel mixture alternating between clean and rich conditions. During operation with a lean air/fuel mixture, the barium carbonate which is present, for example, in the catalyst material of the nitrogen oxide storage catalytic converter removes nitrogen oxide (NOx) from the exhaust gas, which is at that time oxidizing, to form solid barium nitrate. On account of the associated load imposed on the material, from time to time it is necessary to regenerate the NOx storage catalytic converter. This process, which is known as nitrate regeneration, is effected by operating the internal combustion engine with a rich air/fuel mixture for a certain time. In the process, the barium nitrate, which is unstable in the resulting exhaust gas containing reducing agent, decomposes again to form barium carbonate and to release NOx. The latter is then reduced by the reducing agents (H2, CO and HC) present in the exhaust gas, at the precious metal component which is applied to the NOx storage catalytic converter, predominantly to form harmless nitrogen (N2).
In German patent document DE 101 13 947 A1, the regeneration of a nitrogen oxide storage catalytic converter is initiated when a predetermined threshold value for the nitrogen oxide concentration in the exhaust gas on the output side of the nitrogen oxide storage catalytic converter is exceeded. In this case, the regeneration comprises a first phase, in which the air/fuel mixture fed to the internal combustion engine is comparatively greatly enriched, and a second regeneration phase following the first regeneration phase, in which the air/fuel mixture fed to the internal combustion engine is comparatively less enriched.
Accordingly, lowering the levels of NOx over a prolonged period using the above method requires alternating the operation of the internal combustion engine between lean and rich conditions. It should be noted, however, that the rich-burn operation required for the nitrate regeneration operations diminishes the benefit that is achieved in terms of fuel consumption by lean burn operation of the internal combustion engine. Therefore, with a view to fuel consumption, it is desirable for the proportion of time taken up by lean-burn operation to be as high as possible, and therefore that the regeneration to be as short as possible. On the other hand, it is desirable for the regeneration of the nitrogen oxide storage catalytic converter to be as complete as possible so that, after regeneration has taken place, the storage catalytic converter is capable of storing as much nitrogen oxide as possible. Nevertheless, for emission reasons, a breaking through of harmful reducing agents should be avoided.
Therefore one object of the invention is to provide a method for regenerating a nitrogen oxide storage catalytic converter as efficiently and effectively as possible.
This and other objects and advantages are achieved by the method according to the invention, in which a regeneration is triggered when a triggering threshold value for the nitrogen oxide concentration in the exhaust gas on the output side of the nitrogen oxide storage catalytic converter is exceeded. Initially, a first regeneration mode with a constant air/fuel ratio λM of the air/fuel mixture burned in the internal combustion engine is set. Following the first regeneration mode, according to the invention a second regeneration mode with a variable value for the air/fuel ratio λM is set. In the second regeneration mode, the time rate of change d λM/dt of the air/fuel ratio λM is set as a function of either the mass flow of the exhaust gas flowing through the nitrogen oxide storage catalytic converter, or an internal combustion engine operating variable linked with the mass flow of exhaust gas.
The air/fuel ratio, also referred to as the lambda value, is understood here, in the usual way, as meaning the stoichiometry ratio of the content of oxygen and the content of fuel or of reducing components in the air/fuel mixture fed to the internal combustion engine or in the exhaust gas. The designation λM is selected below for the air/fuel ratio of the air/fuel mixture fed to the internal combustion engine. In this case, during the regeneration of the air/fuel mixture fed to the internal combustion engine, a lambda value of λM≦1.0, (that is, a stoichiometric or reducing air/fuel mixture) is preferably set.
The manner in which the time rate of change d λM/dt of the air/fuel ratio λM depends on the mass flow of the exhaust gas flowing through the nitrogen oxide storage catalytic converter or on an internal combustion engine operating variable linked with the mass flow of exhaust gas, is preferably selected in such a manner that given a comparatively small mass flow of exhaust gas, the nitrogen oxide storage catalytic converter in the second regeneration mode is fed, with an exhaust gas having a temporally rising content of reducing agent and, given a higher mass flow of exhaust gas, it is fed with an exhaust gas having a temporally decreasing content of reducing agent. In addition, the dependency is preferably selected in such a manner that, at customary driving states of the associated motor vehicle, a gradually rising lambda value is produced over the course of the second regeneration phase.
In this manner, it is taken into account that, as the regeneration continues, the demand for reducing agent gradually decreases. An excess of reducing agent supplied and a resulting leakage of reducing agent are therefore also avoided. Since a decreasing lambda value is set when there is a small mass flow of exhaust gas, the length of time that the reducing agent spends in the volume of the catalytic converter increases when there is a small mass flow of exhaust gas, and the reducing agent can therefore be completely converted even at high concentration, thus avoiding leakage of the reducing agent.
In a refinement of the invention, the first regeneration mode is ended after a predeterminable first period of time. In the first regeneration mode, a comparatively low air/fuel ratio of approximately λM=0.8 is set. The period of time for maintaining the first regeneration mode (first regeneration phase) is also dependent on the volume of the nitrogen oxide storage catalytic converter and is preferably selected to be comparatively short (for example, approximately one second). The period of time and the lambda value of the first phase of the regeneration of the nitrogen oxide storage catalytic converter, if the latter still has a comparatively large amount of nitrogen oxides or oxygen stored in it, is preferably selected such that a large part of the stored nitrogen oxides or of the stored oxygen is already reduced, thus avoiding leakage of reducing agent. The selection of predeterminable and preferably fixedly applied values for the duration and the air/fuel ratio in the first regeneration phase takes account of the fact that, after the lean-burn storage phase ends, a minimal amount of nitrogen oxides is stored in the nitrogen oxide storage catalytic converter.
In a further refinement of the invention, the second regeneration mode is ended after a predeterminable second period of time. The second period of time is preferably fixedly applied and selected in such a manner that, taking the storage capacity of the nitrogen oxide storage catalytic converter into account, the majority of the stored nitrogen oxides is reduced when this regeneration phase ends.
In a further refinement of the invention, in a third regeneration mode, the time rate of change d λM/dt of the air/fuel ratio λM is set as a function of the mass flow of exhaust gas or as a function of both an internal combustion engine operating variable linked with the mass flow of exhaust gas and the measured value of a lambda probe arranged in the exhaust pipe on the output side of the nitrogen oxide storage catalytic converter. In this case, a lambda probe is understood as meaning a sensor which supplies a signal dependent on the lambda value of the exhaust gas. An NOx sensor, preferably with lambda functionality, can likewise be used. By additionally taking into consideration the lambda value of the exhaust gas present on the output side of the nitrogen oxide storage catalytic converter, the regeneration progress can be particularly reliably detected and taken into consideration by the consequent setting of the air/fuel ratio of the internal combustion engine. An oversupply of the nitrogen oxide storage catalytic converter with reducing agents and an associated leakage of reducing agent can therefore be avoided. This is particularly important toward the end of the regeneration when only small amounts of nitrogen oxide are still stored in the nitrogen oxide storage catalytic converter.
The third regeneration mode may be set instead of the second regeneration mode, but, according to a further refinement of the invention, the third regeneration mode is preferably set directly after the second regeneration mode ends.
In a further refinement of the invention, the setting of the air/fuel ratio λM is limited to a value range with a predeterminable lower limit value λmin and a predeterminable upper limit value λmax. This measure firstly makes it possible to avoid too sharp a drop of the air/fuel ratio and therefore a leakage of reducing agent. Secondly, it is avoided that the air/fuel ratio rises too severely and thereby, under some circumstances, the rich range preferred for the regeneration is even exceeded and hence regeneration no longer takes place. Preferably, when the lower limit value λmin is reached, the air/fuel ratio is kept at the lower limit value until a rise of the air/fuel ratio is initiated again by the mass flow of exhaust gas rising. Correspondingly, it is preferably provided, when the upper limit value λmax for the air/fuel ratio is reached, to keep the latter at this limit value until a dropping of the air/fuel ratio is initiated again by the mass flow of exhaust gas dropping.
In a further refinement of the invention, the triggering threshold value for triggering the regeneration of the nitrogen oxide storage catalytic converter is predetermined and/or the time rate of change d λM/dt of the air/fuel ratio λM is set as a function of an aging factor representing the aging of the nitrogen oxide storage catalytic converter. The aging factor representing the aging is preferably derived from the current nitrogen oxide storage capacity of the nitrogen oxide storage catalytic converter and comparison with the nitrogen oxide storage capacity of the nitrogen oxide storage catalytic converter in the unaged state. The current nitrogen oxide storage capacity can be determined, for example, by measuring leakage of nitrogen oxide during the lean storage phase and comparing it with the raw emission of nitrogen oxide from the internal combustion engine. In this case, it is advantageous to determine the storage capacity of the nitrogen oxide storage catalytic converter with predeterminable reference conditions, for example with regard to speed of rotation, load and/or exhaust gas temperature, and to compare it with a reference value, determined beforehand under the same conditions, of the unaged nitrogen oxide storage catalytic converter.
By matching the triggering threshold value to the aging state of the nitrogen oxide storage catalytic converter, aging-induced reduction of the nitrogen oxide storage capacity can be reacted to. Preferably, as the nitrogen oxide storage catalytic converter increases in age, the triggering threshold value is lowered. The regeneration operations therefore take place at shorter intervals with which the lower storage capacity is taken into account. By means of the aging-dependent setting of the time rate of change d λM/dt of the air/fuel ratio λM in the second or in the third regeneration phase, the aging-induced reduced amount of stored nitrogen oxides can be reacted to and the regeneration correspondingly adapted. Preferably, as the nitrogen oxide storage catalytic converter increases in age, a greater change of the air/fuel ratio λM can be provided at a certain mass flow of exhaust gas, so that the duration of the regeneration is shortened.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.